This invention relates to methods for providing compressed air to be used for breathing by humans. Such air is frequently contaminated with organic vapors and carbon monoxide which must be eliminated or reduced to levels considered safe for human consumption. Maximum permissible concentrations of contaminants in breathing air are specified by the OSHA and the Compressed Gas Assoc., Inc.
Carbon monoxide is a particularly troublesome contaminant to remove from breathing air because it is not absorbed by charcoal and the common desiccants like silica gel, activated alumina and molecular sieves. In prior art systems carbon monoxide is typically eliminated or reduced to acceptable levels by passing this air through suitable catalysts such as Hopcalite material which converts CO in the air into CO.sub.2.
Hopcalite and similar catalysts, however, are catalytically inactive at moisture levels of ambient air. As noted in Razette U.S. Pat. No. 4,089,655 (column 4, lines 11-15) these catalysts will remove CO by conversion to CO.sub.2 only when there is relatively dry air in contact with the catalyst. To achieve the necessary dryness in the air the prior art has typically used desiccants upstream of the catalyst as taught, for example, in the Razette patent at column 3, lines 53-55.
Such systems have several deficiencies. The most serious one is the limited life of the desiccant causing frequent shutdowns to replace the desiccant. More importantly, if the desiccant is not promptly replaced the catalyst ceases to function increasing the likelihood of life threatening contamination of breathing air. Another deficiency is the chance that the system will become swamped with surges of contaminants that can occur when an air compressor overheats, or when chemicals are spilled near the air intake of the compressor. The capacity of both desiccant and charcoal adsorbents are likely to be exceeded under these circumstances. Another disadvantage of desiccant systems is the dryness of breathing air produced. For instance, air from the desiccant cited in U.S. Pat. No. 4,089,655 reduces the dew point down to -70.degree. F. This would seriously dehydrate the membranes of workers using the breathing air. Also, in some desiccant systems part of the breathing air is diverted and used to dry the desiccant which, of course, reduces the amount of breathing air available to the worker.
While other means of drying compressed air are taught in the prior art these means have not been adapted to the particular problems associated with preserving the operating efficiency of catalysts. For example, Huffman U.S. Pat. No. 3,585,808 discloses a compressed gas dryer utilizing a refrigeration cycle to first cool and then warm compressed gas but does not disclose nor appreciate how to condition compressed air that is to be passed through a catalyst. Similarly, Fricke U.S. Pat. No. 2,096,851 discloses use of an expansion valve to dry compressed air but contains no teaching directed to utilization of that air in conjunction with catalysts.